When gutters haven’t been cleaned in a while they tend to fill up with dirt, leaves, roofing shingle material, and other debris, rendering the gutters useless and unable to do their intended job. Which is to safely evacuate water from your home or structures roof line.
Gutters left uncleaned can lead to a whole host of problems including but not limited to, roof damage, drywall and insulation rot, and if left unchecked long enough, water damage has been known to travel all the way to the foundation.
Most homes should have their gutters cleaned out at least once a year. But, depending on tree cover, maybe multiple times a year.